Well, you can't exactly say the old denizens of the British Isles had THAT high an opinion of the Fair Folk. I don't know if leprechauns were ALWAYS of the Seelie Court, though. In other words, were UNseelie leprechauns possible?

God is a gateway delusion. It might seem innocent at first, some Christian or Muslim approaches you about their religion, masking it as just believing in this thing that created you and the whole universe that they call God. But delusions are a slippery slope and once you start believing in one supernatural entity for no reason what's to stop you. Pretty soon you start believing in ghosts and demons, metaphysical planes that no one has ever been to. Before you know it you're teaching lessons in a private Christian school in Louisiana on how the Loch Ness monster proves creationism.